The thermodynamics of interaction between cholesterol and lysophospholipids containing varying fatty acyl chains and polar head groups will be investigated. In addition, the binding of cholesterol to the protein-phospholipid complexes listed below will be studied in order to elucidate the nature of serum lipoproteins found in vivo. 1. Apo AI from human serum high density lipoprotein plus diacyl phosphatidyl cholines. 2. APo AII from human serum high density lipoprotein plus diacyl phosphatidyl cholines. 3. Apo B from human serum low density lipoprotein plus diacyl phosphatidyl cholines. The structural changes of the proteins at various levels of ligand binding will be monitored by circular dichroism and hydrodynamic measurements.